Telephone communication systems typically alert a customer to an incoming call by means of a ringing signal and provide other information through dial tones or busy signals. A suitable reliable AC ringing signal generator is required to supply a signal to drive a ringer or tone source to create the desired alerting or information signal. Such AC ringing signal generators have included rotary generators, magnetic generators and electronic oscillators.
A typical telephone ringer includes a series parallel magnetic circuit with an armature driving a bell clapper which is energized by a winding driven by the ringing signal generator. A certain minimum current is required in that winding to provide sufficient magnetomotive force to drive the bell clapper device. The waveform of this current is asymmetrical and due to the nonlinear magnetic circuit and tends to be highly peaked in one polarity direction. Due to reactive impedances in the ringing load, the current is out of phase with the voltage. Bidirectional power flow is required.
The ringing signal generator normally supplies a 20 Hz driving signal with some requirements for a 60 Hz driving signal for busy signals and dial tones. At these frequencies magnetic and rotary generators are very heavy and have considerable bulk. Electronic ringing signal generators are smaller but tend to have very low efficiency or be very expensive because of the need to power the nonlinear loads provided by the telephone network.
A typical magnetic ringing signal generator uses non linear magnetics and combines and subdivides frequencies to obtain suitable ringing signals. An electronic ringing signal generator normally comprises a ringing signal oscillator driving a ringing signal class B amplifier. These class B amplifiers have tended to be very power dissipative and must be overdesigned because of the asymmetrical nonlinear load that must be driven. Prior art switching power amplifiers have tended to be expensive complex circuits with a very high parts count and have a relatively low reliability.